Abstract
Survival is a key demographic component that often varies as a result of human activities such as recreational harvest. Detailed understanding of seasonal variation in mortality patterns and the role of various risk factors is thus crucial for understanding the link between environmental variation and wildlife population dynamics and to design sustainable harvest management systems. Here, we report from a detailed seasonal and cause‐specific decomposition of mortality risks in willow ptarmigan (Lagopus lagopus) in central Norway. The analyses are based on radio‐collared (n = 188) birds that were monitored across all seasons, and we used time‐to‐event models for competing risks to estimate mortality patterns. Overall, annual survival was estimated at 0.43 (SE: 0.04), with no distinct difference among years (2015/16 to 2018/19) or between sexes. Analysis of mortality risk factors revealed that on the annual basis, the risk of harvest mortality was lower than the risk of dying from natural causes. However, during the autumn harvest season (September–November), survival was low and the dominating cause of mortality was harvest. During winter (December–March) and spring seasons (April–May), survival was in general high and did not vary between males and females. However, during the spring season, juveniles (i.e., birds born last year) of both sexes had lower survival than adults, potentially because they are more prone to predation. During the summer season (June–August), females experienced a higher hazard than males, underlining the greater parental investment of females during egg production, incubation, and chick rearing compared to males. Our analyses provide unique insight into demographic and seasonal patterns in willow ptarmigan mortality risks in a harvested population and revealed a complex interplay across seasons, risk factors, and demographic classes. Such insight is valuable when designing sustainable management plans in a world undergoing massive environmental perturbations.
Highlights
Population dynamics are driven by temporal and spatial fluctuations in demographic rates that together determine the population growth rate λ (Sæther & Bakke, 2000; Sæther, Ringsby, Bakke, & Solberg, 1999)
Personal communication, June 25, 2020), based on estimated population size and total bag size, in our study region in Lierne Municipality. These studies indicate that higher harvest rates yield lower annual survival of willow ptarmigan, which further demonstrate that harvest mortalities are at least partially additive to natural mortalities
We found a distinct difference in survival between males and females during summer, with female willow ptarmigan having markedly lower survival compared to males
Summary
Population dynamics are driven by temporal and spatial fluctuations in demographic rates that together determine the population growth rate λ (Sæther & Bakke, 2000; Sæther, Ringsby, Bakke, & Solberg, 1999). After a strong decline in population numbers, the willow ptarmigan was in 2015 classified as near threatened (NT) in the Norwegian Red List for Species (Henriksen & Hilmo, 2015), fueling a debate of harvest effects on population development (Breisjøberget, Odden, Storaas, Nilsen, & Kvasnes, 2018) This makes the Norwegian willow ptarmigan population a highly relevant case study for a detailed examination of variation in mortality patterns for a managed wildlife species. To this end, we used 5 years of telemetry data from central Norway to characterize annual and seasonal mortality risks for different sex- and age classes. These analyses will provide an important description of how different hazards shape the annual mortality patterns for different demographic groups in a wildlife population
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